Many vehicles are provided with a steering control such as a wheel or yoke which allows the driver to control the vehicle's direction. Taking a conventional automobile as an example, it is typically equipped with a steering wheel which is located in front of the driver. A control linkage extends forwards from the steering wheel to a mechanism such as a steering rack that converts rotation of the steering wheel to an appropriate motion of the automobile's steering wheels. The control linkage, together possibly with any cosmetic or support structures associated with it is known as a steering column. In addition to automobiles, similar structures exist in other vehicles, such as trucks, motor boats and aircraft.
The steering column typically extends away from the driver's position, most normally forward of the driver. If the vehicle is involved in a collision the driver's body might hit the steering column. It is therefore desirable for the steering column to be able to deform, particularly by collapsing in the direction along its axis, so as to absorb energy and reduce injury to the driver.
Several techniques are known for absorbing kinetic energy in a steering column. In one class of techniques a shaft of the steering column is capable of telescoping axially, a first part of the column being in the form of a tube into which a second part of the column can slide. Relative motion of the first part relative to the second part is resisted by means of a frictional clamping arrangement. When the column is subjected to an axial compression force that is high enough to overcome the friction of the clamp the two parts can collapse telescopically. One problem with this arrangement is that it is difficult to design the clamp so that energy is absorbed evenly as the column collapses. Once the frictional force of the clamp has been overcome the collapse of the steering column assembly can sometimes occur with minimal resistive force.
In a typical implementation of a steering column assembly in a vehicle, the assembly is secured to a support structure by means of a support bracket. The support structure may form part of the vehicle chassis or be some other structural component of the vehicle. In another class of energy absorption techniques energy is absorbed due to work done in the plastic deformation of the support bracket, or of other intermediary structures linking the steering column to the structure of the vehicle.
EP 0,479,455 B1 (Melotik) is an example of such a steering column assembly. In Melotik a support bracket connects a steering column assembly to a support structure. FIG. 1 is a diagram of the support bracket. The right-hand portion of FIG. 1 shows the bracket in its-non-collapsed state (i.e. before a substantial impact) and the left-hand portion of FIG. 1 shows it in its collapsed state. The support bracket is broadly U-shaped in plan-view, and contains a base portion 54 and two side members 56 and 58. The support bracket is designed to fit around the steering column such that the side members 56 and 58 are located on respective sides of the steering column. The base portion contains a bore 62 through which a steering shaft passes. Each side member contains a guide slot 92 which has at its rearward end a pocket 94. When the assembly is in a non-collapsed state (on the right-hand side of FIG. 1) a bolt 70, which connects the support bracket to the structure of the vehicle, sits in the pocket 94. Forward of the pocket the guide slot diminishes in width to be narrower than the diameter of the bolt 70. The guide slot divides the side member into a guide rail 96 and a deformable rail 98. The guide rail is thicker than the deformable rail.
When a significant impact occurs, the bolt 70 is driven forwards from its initial position in pocket 70. As the bolt moves through the guide slot 92 the energy absorbing side rail undergoes plastic deformation. Once the bolt reaches the end position, as illustrated on the left-hand side of FIG. 1, the guide slot will have been deformed to a slot of uniform width. Energy is absorbed by the support bracket as the deformable rails are pushed sideways. Although in theory it might be possible to design the deformable rails so that energy can be absorbed uniformly along the travel of the bolt, in practice this would be expected to be difficult. The reason for this is that when the bolt is part way through its travel parts of the deformable rails will already have moved sideways, and that movement will significantly affect the force needed to advance the bolt further into the slot.
EP 2,377,743 (Olgren) discloses a further method of absorbing the kinetic energy of an impact onto the steering column by means of plastic deformation. The steering assembly of Olgren comprises a steering shaft housed in a jacket. A support bracket attaches the assembly to the vehicle. FIG. 2a is a diagram of the support bracket 30. The bracket contains a substantially horizontal base plate 54 and two vertical panels 56 and 58 which project downwards from the base plate. Each vertical panel contains a first slot 60, the slot extending horizontally.
In one structure disclosed by Olgren a carriage is attached rigidly to the jacket of the steering shaft, the carriage being supported by the bracket 30. FIG. 2b is a diagram of the carriage 32. The carriage is generally U-shaped when viewed in cross-section and contains two vertical walls 64 and 66. Each vertical wall contains a guide slot 68 which has at its one end a hole 100. The width of the guide slot is uniform along its length and less than the diameter of the hole 100. A bolt is used to assemble the carriage to the bracket 32. When the carriage is assembled to the bracket 32 and the steering column assembly is in a non-collapsed state the bolt is threaded through the holes 100 of the carriage and slots 60 of the support bracket 30. The diameter of the bolt is greater than the width of the guide slot 68. During the collapse of the assembly the bolt travels from the hole 100 through the guide slot 68 to an end position 106. Because the diameter of the bolt is greater than the width of the guide slot, the bolt causes the vertical walls 64 and 66 to plastically deform. The work done in plastically deforming the vertical walls absorbs a portion of the impact energy of the driver onto the steering column assembly.
In another structure disclosed by Olgren energy is absorbed through the plastic deformation of the support bracket itself. In this embodiment the slots of the support bracket are contoured in the same fashion as the guide slots 68 of the carriage in the first embodiment. FIG. 3 is a diagram of the support bracket according to the second embodiment. A bracket 230 comprises a substantially horizontal base plate and two vertical panels that extend vertically downwards from the base plate. Each panel has a guide slot 260, with a hole at its one end. The guide slots have uniform width which is less than the diameter of the hole. When the bracket is in its initial assembled position a bolt 280 threads both holes. The diameter of the bolt is greater than the width of the guide slots. In the event of a substantial impact the bolt is driven along the slot 260. Energy is absorbed by the plastic deformation of the bracket 230.
There is thus a need for an improved method of absorbing kinetic energy during the collapse of a steering column.
According to the present invention there is provided a collapsible steering column assembly comprising a mounting structure, the mounting structure comprising a first part for connecting to the body of a vehicle and a second part for connecting to part of a steering mechanism, the first and second parts being interconnected to permit relative movement therebetween as the assembly collapses, wherein: one of the first or second parts comprises a slot, the slot comprising a pocket and a channel defined by two opposing sidewalls, each sidewall being the outer edge of a deformable structure; the other of the first or second parts comprises a lug, the lug extending into the pocket when the assembly is in a non-collapsed state and being configured to be driven through the channel to cause plastic deformation of the deformable structure as the assembly collapses; and the assembly comprises a re-enforcement that limits the plastic deformation of the deformable structure during the collapsing stroke to regions of the deformable structure adjacent to regions of the channel through which the lug has passed.
Suitably the deformable structure comprises a relatively strong portion running longitudinally with the channel and a relatively weak portion running longitudinally with the channel and located between the relatively strong portion and the channel.
Suitably the relatively strong portion of the sidewall is thicker than the relatively weak portion of the sidewall when viewed in cross-section in a plane perpendicular to the longitudinal direction of the slot.
Alternatively the relatively weak portion could have a tapered profile when viewed in cross-section in a plane perpendicular to the longitudinal direction of the slot.
The relatively strong portion may have a higher material hardness than the relatively weak portion. The relatively strong portion may have a greater cross-sectional area than the relatively weak portion. The relatively strong portion could be configured with strengthening structures, such as ribs, which contribute to it being more resistant to deformation than the relatively weak portion. The relatively weak portion could be configured with zones of weakness, such as grooves, notches or perforations, which contribute to it being less resistant to deformation than the relatively strong portion.
Preferably the re-enforcement is the relatively strong portion.
Preferably the relatively strong portion is configured such that plastic deformation of the deformable structure is limited to the relatively weak portions.
Suitably the total width of the channel and the relatively weak portions is at least as great as the width of the lug.